Chapter 2 Flashcards
How was gas found in the early days
Oil was found based on visual monitoring. Gas seeps at the surface or accidentally finds during drilling for water.
What is Geophysics
The science of studying the physical processes and physical properties of Earth and its surrounding environment and utilization of quantitative methods for their analysis.
Objective is to locate or detect the presence of subsurface structures or bodies and determine their size, shape, depth, and physical properties.
What does it mean to be passive or active when determining if the area contains oil?
Passive: Using the methods of natural fields of earth to carry out analysis
Active: Using artificial methods to carry out analysis
What are the 5 geophysical methods and one additional method engineers are able to get a hand on sub surface and if there is a hydrocarbon rersevoir and the characteristics/worth of it.
Gravity
Magnetic
electromagnetic
Electrical surveying
seismic surveying
well logging
Describe how we would use gravity to determine if there is oil underground.
The gravitational field is caused by the variation in the densities of subsurface rocks
Measures the change in acceleration due to gravity
the gravity anomalies which are the deviation from the geoid as a pre-defined reference level will be reported
Geoid is a surface over which the gravitational field has the same value
Describe how we would use magnetic to determine if there is oil underground.
Investigates the subsurface geology by measuring the strength by measuring the strength or intensity of the Earth’s magnetic field
This measures the magnetic anomalies which are the deviations from Earth’s magnetic field.
These magnetic anomalies are caused by lateral variation in magnetic susceptibility
Magnetic susceptibility is a dimensionless property which is a measure of how susceptible a material is to becoming magnetized.
Describe how we would use electromagnetic to determine if there is oil underground.
A rock that contains water has more electromagneticvity
use the response of the ground to the propagation of incident alternating electromagnetic waves, made up of two orthogonal vector components, an electrical intensity (E) and a magnetizing force (H) in a plane perpendicular to the direction of travel.
A hydrocarbon filled reservoir will typically have high resistivity compared with shale and a water filled reservoirs. This method therefore has the unique potential of distinguishing between a hydrocarbon filled and a water filled reservoir.
Is we put eletrcity into a well we can tell by the resistivity if there is water or oil.
What are the drawbacks of using electromagnetic methods
Results may be confused by non-economic sources such as water bearing zones.
Depth is too low
Ratio can be off and hard to determine
Describe how we would use electrical surveying to determine if there is oil underground.
Employ direct currents or low frequency alternating currents to investigate the electrical properties of the subsurface.
Put into the ground and the resulting potential differences are measured at the surface. Deviations from the pattern of potential differences for homogeneous ground provide information on the form and electrical properties of subsurface inhomogeneities.
What are the drawbacks of using electrical surveying methods
Limited to simple structural configuration. Deviation is a problem
Depth
The effects of near-surface resistivity variations can mask the effects of deeper variations.
Interpretations are ambiguous
Describe how we would use seismic surveying to determine if there is oil underground.
seismic waves are created by a controlled source and propagate through the underground layers.
these travel times may be converted into depth values and subsurface interfaces will be mapped based on the results.
At every layer it travels at different speeds, we can then get an idea of what’s underground
Describe how we would use well logging to determine if there is oil underground.
A well is drilled
Identifies hydrocarbon zones, depth, composition, orientation and other properties rock/fluid mixtures
Logging tools
Electrical
Radioactivity
Acoustic
What are 3 main parameters that define the characteristics of a reservoir
Permeability, porosity, water saturation
What defines storage capacity and transmibility of a reservoir
porosity and permeability
The information from a seisimic survey indicates…
The types of rock, their relative depth, and whther a trap is present
What is the equation used to determine porosity
Vp / (Vp + Vg) times 100%
Vp = pore or void volume
Vg = grain volume
Vb = bulk volume of rock
Define Absolute porosity
the ratio of the volume of all pores
(interconnected or not) to the bulk volume of the rock.
Define Effective porosity
the ratio of interconnected pore volume to the
bulk volume of the rock (This is the value which is used in
reservoir engineering calculations).
Define Primary porosity
porosity in a rock due to
sedimentation process.
Define secondary porosity
porosity in a rock due to the
changes which happen after sedimentation process, Ex. fracturing
and re-crystallization.
What are types of porosity measurements when using well logging
The sonic log
The density log
The neutron log
What are types of porosity measurements in the lab
Boyles law porosimeter
Wet and dry weight method
summation of fluids
What is the sonic log equation
θsl = ∆t(sl) - ∆t(ma) / ∆t(f) - ∆t(ma)
∆t(sl) is the value of the acoustic transit time measured by the sonic log, usec / ft
∆t(ma) is the value of the acoustic transit time of the rock matrix measured in the lab, usec / ft
∆t(f) is the value of the acoustic transit time saturating fluid measurements in the lab, usec / ft
θsl is the porosity from the sonic log (log measurements), fraction
What does saturation mean?
fraction, or percent, of the pore volume occupied
by a particular fluid (oil, gas, or water)
What is the fluid saturation equation
fluid saturation equation = total volume of the fluid / pore volume
What does So, Sg, Sw represent
So = oil saturation
Sg = gas saturation
Sw = water saturation
Does the reservoir rock show larger variations in porosity when vertical or horizontal to the bedding planes
Vertically
If the porosity in one portion of the reservoir is greatly different from that in another area. What two equations should we use?
areal weighted average or volume weighted average
What does n, hi, θi, and Ai represent in the two equations - areal weighted average and volumetric weighted average
n = total number of core samples
hi = thickness of core samples i or reservoir area i
θi = porosity of core sample i or reservoir area i
Ai = reservoir area i
What is the difference between arithmetic average and thickness weighted average
Arithmetic is the sum divided by the total
Thickness-weighted average is each measurements times the thickness then all divided by the sum of the thickness.
Define critical oil saturation
For the oil phase to flow, the saturation of the oil must exceed a certain value which is called critical oil saturation. Like oil still stuck in a container
Define residual oil saturation
There is oil, but the technology is not good enough to keep producing the oil economically. That makes up 30% of the oil
Define connate water saturation
the water that was born with the rock
is the saturation
of water on discovery of the reservoir
Define initial water saturation
The changes the rock has gone under over time after the initial connate water saturation. Contains the later stage of water saturation
Define irreducible water saturation
Water cannot flow into the rock anymore
Define permeability
Permeability is the ability of the rock to transmit fluid.
Permeability depends on how the pore spaces are
interconnected
What is the unit for permeability
1Darcy = 1000 md, 1 Darcy = 0.987 x 10-12 m2
What is the equation to calculate permeability?
Q = - (A * ∆P * k) / (∆x * u)
For the equation of permeability, what does each value represent?
Q = - (A * ∆P * k) / (∆x * u)
Q = flowrate (cm^3 / sec)
K = permeability (darcy)
A = cross section area (cm^2)
u = fluid viscosity (cp)
P = pressure (atm)
L = length (cm)
What are Darcy’s 5 law assumptions
1)A gas phase is not present. For this simplified analysis, it is assumed that
no water is present. Therefore, the flow has a single-phase.
2)The undersaturated oil is assumed to behave as an incompressible fluid.
3)The characteristics of reservoir rock are assumed constant for all
orientations.
4) The flow is assumed to be at steady state.
5)The pore size is small such that the Reynolds Number is small and the
flow is laminar.
5)The pore size is small such that the Reynolds Number is small and the
flow is laminar.
What values of Darcy’s equation do we use as the vertical and horizontal axis to yield a straight line on a graph?
Q/A - vertical vs dP/dL horizontal
or
Q - vertical vs ∆P
for a graph of permeability, what values of Darcy’s equation is the slope?
slope = k / u = mobility ration
What is Darcy’s law for inclined intervals?
Q = - (A/u) (∆P/L +- pgsinΘ)
When do we use Q = - (A/u) (∆P/L +- pgsinΘ) as the plus or negative version?
- when Q(the flow) is pointing uphill
+ when Q is flowing downhill
What is Darcy’s law for radial flow?
Q = 2πkH(Pe - Pwf) / uln(re/rw)
What is the process to measure permeability in a rock from a well
A core goes into a sleave and then into a core holder. There will be a bunch of pressure ports connected to the holder. There are valves, and one is connected to water (brine) supply. Then we vacuum the system to –1 atm, get the air out for half a day. Then they measure the permeability. The brine goes through the interconnected pores in some random path. A flow meter will record the flow. Then they make a table comparing Q and change of P
what does it mean to have permeability in multilayered reservoirs? and which permeability do we calculate for?
Layered parallel beds with different permeabilities – we then find the average permeability (Transmissibility: thickness * L = width)
when we have permeability in multi-layered reservoirs what do we set our equations to when they are in series? and in parallel?
Series - Q
Parallel - ∆P
Define effective permeability
As the saturation of a particular phase decreases, the
permeability to that phase also decreases. The measured
permeability is referred to as the effective permeability
and is a relative measure of the conductance of the
porous medium for one fluid when the medium is
saturated with more than one fluid.
Equation for relative permeability
Kg + Ko + Kw <= K
Define relative permeability
Relative permeability is defined as the ratio of the
effective permeability to a given fluid at a definite
saturation to the permeability at 100% saturation.
How to calculate the relative permeability of oil
Kro = Ko/K
Where Ko is the effective permeability of oil
How to calculate the relative permeability of water
Krw = Kw/K
Where Kw is the effective permeability of water
How to calculate the relative permeability of gas
Krg = Kg/K
Where Kg is the effective permeability of gas
What are the 3 types of rock compressibility and the name of the one that encompasses them all
- Rock-matrix compressibility
- Rock-bulk compressibility
- Pore compressibility
Formation compressibility encompasses all 3
Define wettability
the tendency of one fluid to spread on or
adhere to a solid surface in the presence of other immiscible
fluids.
What do we measure to determine wettability
Contact angle at the liquid-solid surface
Contact angle of mercury, water, and oil
Relative to 90 degrees
Mercury > 90
oil = 90
water < 90
surface tension vs interfacial tension
When two immiscible fluids are in contact and these two fluids are
liquid and gas, the term surface tension is used to describe the
forces acting on the interface. When the interface is between two
liquids, the acting forces are called interfacial tension.
Gross pay refers to what section of a reservoir
Oil
between GOC and WOC
What does net pay mean
Net Pay is that part of the reservoir thickness which contributes
to oil recovery
What are the limits of net pay
- Lower limit of porosity
- Lower limit of permeability
- Upper limit of water saturation
What are the general forms of geophysics
Passive and Active
What are the limitations of the formation evaluation method in
identifying reservoir characteristics?
This is well logging.
The formation of the rock can be complex and the tools can be inaccurate in predicting what is underneath the ground
What is the major difference between formation evaluation
method and seismic method?
Formation evaluation goes under the subsurface of the ground to evaluate.
Seismic method does not go under the subsurface, all evaluations are taken above ground.
How could you calculate the average saturation in a reservoir
when thickness of the rock and porosity is different at various
depths?
Use the thickness weight average
Is permeability of a rock sample when 100% saturated with oil
and measured with oil different with the case when it is
saturated with water and measured with water at 100%? Explain
why?
Yes, the permeability of a rock sample can differ when it is 100% saturated with oil compared to when it is saturated with water, and this is due to a concept called relative permeability
.
Relative permeability refers to the ease with which a fluid can flow through a porous medium in the presence of other fluids
